The present invention relates to low molecular weight polyesters and polyamides useful for dispersing pigments and for coloring polymers. More specifically, the compositions of the invention are effective for displacing water from aqueous pigment dispersions, producing pigment dispersions and then coloring host polymers with the pigment dispersions.
It is well known in the art to produce pigments. During the manufacture of pigments, an aqueous pigment dispersion is typically produced and it is necessary to flush these aqueous pigment dispersions to remove water therefrom. However, the removal of water by simply drying can result in an unwanted agglomeration of pigment particles. Therefore, in practice, only a portion of the water is removed, leaving a finely divided pigment in the form of a wet cake. It is known in the art to remove residual water from such wet cakes by flushing them with polymeric dispersing agents. The process of flushing prepares dispersions in which the pigments remain finely divided and in an intimate mixture with a pigment dispersing agent for further processing such as incorporation into polymeric compositions and inks. The process of flushing pigments is described in U.K. Patent 915,453.
It is also well known to color high melting point polymers. U.K. patent 1,044,378 teaches uniformly dispersing a pigment in a high melting point polymer by forming an intimate mixture of a pelletized or powdered pigment with a pelletized or powdered polymer compatible with the high melting point polymer but having a lower melting point than the high melting point polymer. The process involves heating and mixing to uniformly distribute the pigment in the low melting polymer, solidifying the mass, mixing the solid with a high melting point polymer and melting them together. Each of the high and low melting point polymers are of the same chemical type. U.S. Pat. No. 4,279,802 forms a concentrate of a pigment and a polyester, which concentrate is then used to color a linear polyester. U.S. Pat. No. 5,336,314 uses a polyester or acrylic polymer having phthalimide or naphthalimide groups as pigment dispersion vehicles and pigment presscake flushing agents for the preparation of coatings.
According to this invention, a pigment dispersion is formed by mixing a wet pigment presscake with a low molecular weight polyester or polyamide pigment dispersing agent. The mixture is coagulated and the aqueous phase is removed, typically by heating the composition, optionally in a vacuum. The pigment dispersed in the low molecular weight polymer can optionally be pulverized. The pulverized pigment/dispersing agent mixture can then be diluted with a polar host polymer such as polystyrene homopolymer, styrene copolymers, polycarbonates, polyesters, or polyamides to give high quality color concentrates for eventual manufacture into finished consumer parts and goods. The concentrates are very high quality pigment dispersions, achieving maximum coloration at minimum pigment concentration with efficient use of equipment and machinery. The pigment dispersing agent of the invention is a low melting temperature, low viscosity vehicle which acts as a receptor for the pigment and separates the water during the flushing process. Alternatively, the dispersing agent acts as a low viscosity fluid in the melt state which uniformly wets out the pigment and disperses it by the mechanical action of mixing equipment, such as a single or twin screw extruder. The coloring process is conventionally carried out by blending the flushed, dried and ground pigment containing the dispersing agent with a host polymer and extruding and pelletizing this blend. Satisfactory performance requires that the pigment dispersing agent of choice be compatible with the polymer with which it is subsequently mixed to efficiently disperse the pigments during the coloring process; have good dispersion of the pigment to obtain maximum tint or color intensity and homogeneous distribution of the pigment; and have good extrusion characteristics to produce flexible strands with good strength and pelletizing characteristics for maximum production efficiency and output rate.
It has now been found that by using a certain molar ratio of the dispersing agent precursors, the compatibility of the dispersing agent composition can be tailored to the host polymer. The characteristics of the dispersing agent, such as, melting point and viscosity can be made to fit specific host polymer processing characteristics. In this way, an class of polyester or polyamide dispersing agents with adjustable physical properties can be made compatible with a selected host polymer to suit a particular end use application.